1. Field of the Invention
This invention relates to a system for deriving radiation images. The system is particularly useful in computer, graphics applications.
2. Description of the Related Art
Much work has been done in the computer processing of three-dimensional data to produce realistic two-dimensional pictures. In order to create realistic images, the global illumination which is derived from the interreflection of light within an environment must be modeled correctly. In most conventional models, a constant ambient factor is used as the global illumination term. Shadows and intersurface illumination are not simulated. Although fast, such an approach is not quite satisfactory, particularly where the simulated environment includes complex lighting and object arrangements.
The majority of surfaces in real environments are diffuse reflectors; that is, an incident beam of light is reflected or scattered in all directions within the entire hemisphere above the reflecting surface. A special case of diffuse reflection is the so-called "ideal" diffuse or "Lambertian" reflection. In this case, the incident light is reflected from a surface with equal intensity in all directions. A model assuming ideal diffuse reflection may be adequate for many computer graphics applications.
Methods have previously been developed to determine the exchange of thermal radiant energy between surfaces in an enclosure. See Thermal Radiation Heat Transfer, by Robert Siegel and John R. Howell, Hemisphere Publishing Corp., 1978 and Radiation Heat Transfer, by E. M. Sparrow and R. D. Cess, Hemisphere Publishing Corp., 1978. The application of the same concept to the exchange of optical energy in enclosures, known as the radiosity method in computer graphics, is outlined in the paper "Modeling the Interaction of Light Between Diffuse Surfaces," by Cindy M. Goral et al., ACM Computer Graphics (Proceedings 1984), pp. 213-222. In contrast to conventional methods, the radiosity method models the interreflection of light between surfaces in an environment producing shadows and providing more realistic images, particularly where the environment includes many diffuse surfaces.
Ray tracing techniques have also been developed as a model for light reflections which are assumed to be specular. See "An Improved Illumination Model for Shaded Display" by Turner Whitted in Communications of the ACM, vol. 23, No. 6, June 1980. As in the radiosity method, ray tracing accounts for shadows and interreflection of light between surfaces in an environment and produces more realistic images than conventional methods.
While the radiosity and ray tracing methods provides better images than the conventional method, these techniques require many computer hours using a commercially available mini-computer. The illumination calculations in these techniques may require considerable time since the surfaces visible to a viewer must be tested to determine whether they are in shadow or not with respect to one or more light sources. Hence, much computer time is required to produce one image. It is thus desirable to provide improved radiosity and ray tracing systems which are faster than known methods.
In the conventional, radiosity and ray tracing methods discussed above, a number of steps are required to produce one image of an environment. Frequently, a number of images from different viewing locations may be desired of the static environment. In such event, the steps for producing an image in the conventional and ray-tracing methods must be repeated in their entirety to produce additional images. In the radiosity approach, only some of the steps need to be repeated. In general, it is desirable to provide methods where the steps required for the first image need not be entirely repeated for subsequent images to accelerate the process for producing different images of the same environment.